1. Field of the Invention
This invention concerns a thermal head mounted to a thermal printer and, more specifically, it relates to an improvement in the structure of a glass glaze layer.
2. Description of the Prior Art
A thermal head mounted to a thermal printer comprises, for example, a plurality of heat generating resistance elements arranged linearly on an identical substrate, which are heated by electric supply for developing color and performing recording on a heat sensitive recording paper or transferring and performing recording on a common paper by way of an ink ribbon in accordance with this invention.
A thermal head has been prepared so far, for example, as shown in FIG. 4 by partially forming a glass glaze layer 2 on an insulating substrate 1 made of alumina or the like and further laminating thereover a heat generating resistance layer 3 made of Ta.sub.2 N or the like, an electric power supplying conductor layer 4 made of Al, Ni or the like, an oxidation preventive layer 5 made of SiO.sub.2 or the like and a wear resistance layer 6 made of Ta.sub.2 O.sub.5 or the like successively. In this case, the electric power supplying conductor layer 4 is patterned into a plurality of individual electrodes and a common electrode, between which heat generating portions A are formed. The heat generating portion A is formed at a portion protruded by the glass glaze layer 2, by which a favorable contact with the recording paper or ink ribbon is attained.
Recently, it has been required to the thermal head that recording is possible with a flat surface paper and that a high speed recording is possible. In order to realize the requirements, it is necessary to increase the heat efficiency by improving the heat conductivity of the heat generating portion. Further, the thermal head is also required for such characteristics that the temperature rising in the heat generating portion is fast and that the temperature falling after reaching the maximum temperature is also fast.
When forming the glass glaze layer 2 on the insulating substrate 1 in the conventional thermal head, a paste prepared by mixing a glass frit and binder is coated by printing on the insulating substrate 1 followed by sintering. However, the paste described above, when coated on the insulating substrate 1, extends laterally due to the surface tension failing to effectively protrude the heat generating portion A. Specifically, the glass glaze layer 2 has a height h of about 40 .mu.m and the lateral width w of about 500-700 .mu.m, which is actually extremely flat. Then, the effect of improving the contact of the heat generating portion A is poor failing to improve the heat efficiency sufficiently.
Furthermore, although it may be considered to effectively protrude only the heat generating portion A by restricting the width of the glass glaze layer 2 narrower, the following problems arise in this case. That is, while the glass glaze layer 2 has a function of protruding the heat generating portion A to improve the function of contact, it also has an effect of accumulating heat for increasing the heat generation temperature. If the heat generated from the heat generating portion A is conducted to and dissipated from the heat insulating substrate 1 too rapidly, the temperature of the heat generating portion A can not be increased sufficiently. Therefore, it is required for the glass glaze layer 2 to have a heat capacity to some extent but it is also desired that the temperature falls rapidly once after the heat generating portion has reached a predetermined temperature.